1. Field of the Invention
This invention relates to interfacing digital signals such as, for example, digital audio signals.
2. Description of the Prior Art
Several signal interface formats exist for defining the way in which digital signals are carried (e.g. by cables) from one place to another. Examples include the AES/EBU standard for audio signals, and the so-called xe2x80x9cSDIxe2x80x9d standard for audio/video signals.
These interface formats take into account the physical properties of the signal to be transmitted, such as its bit rate or bandwidth, the identification of components of the transmitted signal, such as the identification of different audio channels within a signal formed of multiple channels, and synchronisation of the received signals so that they can be correctly decoded.
In some interface formats, these matters can impose a large overhead on the data actually carried. For example, in an AES/EBU two channel interface there is a 62.5% overhead imposed to support formatting information. In other words, 62.5% of the data transmission rate is unavailable for carrying actual audio data. Similarly in the MADI 56 channel interface the overhead is 40%.
This invention provides data transmission apparatus for multiplexing one or more channel data streams of equal, predetermined, data rate into a transmission data stream, the apparatus comprising: means for generating a control data stream of the same data rate as the channel data rate, in which first periodically repeating portions of the control data stream provide a synchronisation signal and second periodically repeating portions of the control data stream provide identification data specifying information about the channel data streams with reference to a predetermined ordering of the channel data streams; and means for multiplexing the control data stream with the one or more channel data streams in a cyclic pattern of equally-sized data portions so that a data portion from the control data stream is followed by data portions from each of the channel data streams according to the predetermined ordering of channel data streams.
This invention also provides data reception apparatus for receiving a multiplexed transmission data stream having one or more channel data streams and a control data stream of equal, predetermined, data rate in a cyclic pattern of equally-sized data portions so that a data portion from the control data stream is followed by data portions from each of the channel data streams according to a predetermined ordering of channel data streams, the apparatus comprising: means for detecting first periodically repeating portions of the control data stream representing a synchronisation signal, thereby identifying the control data stream within the transmission data stream; means for detecting second periodically repeating portions of the control data stream providing identification data specifying information about the channel data streams with reference to the predetermined ordering of the channel data streams.
In the invention, a control channel of equal size to one of the data channels is provided to carry formatting and synchronisation information. By multiplexing the control channel and the data channels together in a predetermined cyclic order so that some information to identify the data channels within the data stream is implied, the data overhead of the system can be reduced in comparison to other formats. Indeed, for a two channel interface the invention implies a 33% overhead, much less than that of the AES/EBU format. For a 56 channel interface the invention implies a 1.75% overhead, much less than that of the MADI format.
The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings.